An LC resonant voltage controlled oscillator (VCO) in which variable capacitive elements and an inductor are connected in parallel is one kind of VCOs used in phase-locked loop (PLL) circuits. A conventionally known LC resonant VCO includes a coarse tuning circuit in order to expand the tuning range of the oscillation frequency of the VCO. The coarse tuning circuit enables capacitive elements to be switched in and out of parallel connection with the above-mentioned variable capacitive elements by causing a transistor to be turned on and off, to thereby tune the oscillation frequency.
See, for example, Japanese Laid-open Patent Publication Nos. 2006-33803, 2007-110504, and 2007-158750 and “An 18 mW 90 to 770 MHz synthesizer with agile auto-tuning for digital TV-tuners”, ISSCC 2006/SESSION 11/RF BUILDING BLOCKS AND PLLS/11.1, pages 681-690.
To the source or drain of the transistor of the coarse tuning circuit, a bias voltage based on the on/off state of the transistor is applied in order to prevent damage to the transistor.
However, thermal noise increases with high on-resistance of transistors included in a circuit for generating the bias voltage. The thermal noise is transferred via the capacitive elements of the coarse tuning circuit to the output terminal of the VCO, which then increases the output noise.